DESCRIPTION OF THE PRIOR ART
Stainless steel alloys in the precipitation hardening (PH) class have found ubiquitous application in the aerospace and other high technology industries because of their wide range of mechanical properties. Yield strengths range from 75 to 205 ksi, ultimate strengths from 125 to 220 ksi and elongations from 1 to 25%. Common alloys include the martensitic 15-5PH, semi-austenitic 17-7PH, and austenitic A-286. The martensitic alloy, 17-4PH, has the nominal composition of 17Cr--4Ni--4Cu--2Si--Fe(balance) and has widespread application in aerospace applications.
Stainless steels are typically available in cast or wrought forms but are also available as a powder metallurgy (PM) product. Conventional PM processing of stainless steel includes press and sinter and metal-injection-molding (MIM). Press and sinter results in a compact of only 80 to 85% dense in the sintered condition and is limited to simple geometric shapes such as cylinders. Additional processing such as hot isostatic pressing (HIP) can bring densities to near 100% of theoretical density.
Metal-injection-molding is recognized as a premier forming method for complex shapes, affording significant advantages over other forming methods due to its capability of rapidly producing net shape, complex parts in high volume. Initially, MIM comprised the step of mixing metal powder with a dispersant and a thermoplastic organic binder of variable composition. The molten powder/binder mixture was heated during the injection molding process and injected into a relatively cold mold. After solidification, the part was ejected in a manner similar to injection-molded plastic parts. Subsequently, the binder was removed and the part was densified by a high temperature heat treatment. There were a number of critical stages in this process including the initial mixing of the powder and binder, the injection of the mixture into the mold, and the removal of the organic matrix material. One of the main disadvantages of the initial MIM process is the removal of the organic vehicle. Currently, with organic binder MIM processes the cross section limit of a part for fine particle sizes is typically less than 1/4 inch. If the cross section of the part exceeds that limit, the binder removal process will lead to defects, pinholes, cracks, blisters, etc. Binder removal takes place by slow heat treatments that can take up to several weeks. During debinding at elevated temperatures, the binder becomes a liquid, which can result in distortion of the green part due to capillary forces. Another disadvantage of the initial MIM process is the tendency for the relatively high molecular weight organic to decompose throughout the green body, causing internal or external defects. The use of solvent extraction, wherein a portion of the organic is removed using an organic or supercritical liquid, sometimes minimizes defect formation. Solvent extraction causes difficulties because the remainder still needs to be removed at elevated temperatures, resulting in the formation of porosity throughout the part which allows removal of the remaining organic material. During binder removal, part slumping can pose problems, especially for the larger particle sizes if the green density/strength is not high enough.
MIM offers certain advantages for high volume automation of net shape, complex parts. However, the limitation of part size and excessive binder removal times, along with a negative environmental impact resulting from removal of the organic binder material during the debinding process, have inhibited the expected growth of the use of this technique.
Some improvements, such as the use of water based binder systems, have been made to the initial MIM process. Hens et al. developed a water leachable binder system as described in U.S. Pat. No. 5,332,537. The injection molding feedstock is made with a tailored particle size distribution (to control the rheology), a PVA based majority binder, and a coating on each of the binder particles. During molding, these coatings form necks which give the part rigidity. After injection molding, there is a water debind that lasts several hours. After the remaining binder is cross-linked by either UV or chemical methods, the part undergoes a thermal debind, which takes 8-12 hours for a part such as a golf club head. Other aqueous-based binders contain either polyethylene glycols, PVA copolymers, or COOH-containing polymers. BASF has developed a polyacetal-based system that is molded at moderately high temperatures, after which the binder is removed by a heat treatment with gaseous formic or nitric acid. The acid treatment keeps the debind temperature low to exclude the formation of a liquid phase and thus distortion of the green part due to viscous flow. The gaseous catalyst does not penetrate the polymer, and the decomposition takes place only at the interface of the gas and binder, thereby preventing the formation of internal defects. These improvements are limited by the requirement for separate binder removal furnaces and times, depending on the part size. There are environmental issues as well with removal of the large amount of wax/polymer in the form of fire hazards and volatile organic compound discharge.
An injection molding process using agar as an aqueous binder has been developed by Fanelli et al as described in U.S. Pat. No. 4,734,237. This binder system applies to both ceramic and metal powders. It also includes the use of agarose or derivatives of polysaccharide aqueous gels. The advantage over state-of-the-art wax-based binder technology is the use of water as the fluid medium versus wax. In feedstocks prepared according to this technology, water serves the role of the fluid medium in the aqueous injection molding process, comprising roughly 50 volume % of the composition, and agar provides the "setting" function for the molded part. The agar sets up a gel network with open channels in the part, allowing easy removal of the water by evaporation. By contrast the Hens et al system requires a solvent debind to attain similar open channels in the part. The agar is eventually removed thermally; however, it comprises less than 5 volume fraction of the total formation, and debind times are rapid compared to wax/polymeric debind systems. This is an advantage over the Hens et al system.
This agar-based aqueous binder is especially applicable for the production of stainless steel components using MIM. Due to the easy removal of the aqueous-based binder and its relatively low level of carbon, as compared to wax or polymeric binder systems, sintering schedules can be developed which impart little or no additional carbon to stainless steel alloys such as 316L, 410L and 17-4PH. Excessive amounts of carbon, typically above about 0.07 wt % for 17-4PH, for example, seriously compromise the mechanical properties and corrosion resistance of stainless steels. Moreover, the agar-based binder and its associated carbon are removed in a simple one-step, air debinding process consisting of relatively short debind times of approximately 1/2 to 2 hours. In contrast, wax or polymer based binders require several step debinding processes in which each debind step often takes many more hours. Accordingly, the short air debind times of the agar-based 17-4PH alloy are economically advantageous.